Harmonic Scattering from Passive UHF RFID Tags
Pavel V. Nikitin and K. V. S. Rao
Intermec Technologies Corporation
6001 36th Ave W, Everett, WA 98203
www.intermec.com
Email: {pavel.nikitin, kvs.rao}@intermec.com
Abstract
This paper discusses harmonic scattering from passive UHF RFID tags.
We describe the problem and the basic theory; explain our measurement setup,
and present experimental results for three different commercial Gen2 tags.
Introduction
Currently, there exists a strong interest from aircraft and airline industries
in tagging airplane parts and cabin items using RFID tags. RF emissions from
electronic devices (including various tags) which can interfere with aircraft
systems and components have always been a concern for FAA and have been
extensively studied and regulated [1-2]. Recent FAA RFID policy [3] relieved
passive UHF tags on aircraft from complying with EMC/EMI requirements for
active devices [4]. However, the level of harmonic backscattering from passive
RFID tags remains the subject of interest, especially for environments where
“pollution” of RF spectrum by spurious backscatter emissions is undesirable.
A typical RFID tag consists of an antenna directly connected to an
integrated circuit with nonlinear RF front end. A common front-end architecture
is diode-based voltage multiplier [5] that makes an RFID tag to appear as an
antenna loaded with a nonlinear load. Unlike antenna terminated with constant
impedance load or ideal modulator switch, such system has a nonlinear transfer
function and can backscatter harmonics and intermodulation products when
interrogated with the modulated or unmodulated reader signal [6]:
Vout = α 0 + α 1 Vin + α 2 Vin2 + α 3 Vin3 + ... . ,
(1)
where Vin is the input signal, Vout is the output signal, and α n are harmonic
coefficients. Nonlinear harmonic generation has been extensively studied in
antennas [7] and circuits [8] literature. This phenomenon also found a good use in
harmonic radars [9]. In passive RFID, it was used in active antennas [10], location
estimation [11], and tag radiation pattern measurement [6].
In this paper, we present an experimental characterization of harmonic
scattering from three popular commercial passive UHF RFID Gen2 tags available
on the market today (AD-222, AD-223, and AD-224). Due to our equipment
limitation (highest operating frequency is 2.7 GHz), we concentrate on the
analysis of first three harmonics whose powers are denoted as P1 , P2 , and P3 . The
harmonic frequencies are 880 MHz, 1760 MHz, and 2640 MHz.
Measurement Setup
We employed in our measurements a compact broadband GTEM cell
shown in Fig. 1. Such cells are often used for radiation and susceptibility testing
and allow one to create a controlled uniform field at desired location, excluding
from consideration frequency dependent responses of transmitting antenna and
anechoic chamber material. An RF signal applied to port 1 generates an electric
field inside the cell which can be expressed in terms of input voltage Vin (or
power Pin , both rms), input impedance Z , and cell height h at specific crosssection (the distance between the center conductor and the ground wall) as [12]:
Pin Z
Vin
=
.
(2)
h
h
A specific TEM cell used in measurements was TESCOM TC5060 (S11<-15 dB
in 0.4-3 GHz band). In the tests, an RFID tag was placed inside the cell (where
h=22 cm). Field backscattered from the tag was detected with the sensing probe
antenna connected to the port 2 as shown in Fig. 1. The antenna was simple halfwavelength dipole (no balun) printed on FR4 and located 10 cm away from tag.
E=
Figure 1. Measurement setup with TEM cell: diagram and photograph.
Figure 2. Tags used in measurements, left to right: AD-222 (Impinj Monza 2 IC), AD223 (Impinj Monza 3 IC), and AD-224 (NXP G2XM IC).
The tags used in tests were from AD-22x family by Avery Dennison [13] and are
shown in Figure 2. They had different RFID ICs but similar dipole-like antennas.
In Fig. 1, RF transmitter / RF receiver combo connected to ports 1 and 2
represents one of three equipment configurations used in measurements: a) RF
network analyzer used for S-measurements of ports 1 and 2 (Agilent E5071C); b)
RFID tag tester used for determining the minimum power needed to activate
(read) tags in TEM cell (NI PXI RFID tester described in [14]); c) RF signal
generator and spectrum analyzer used for creating and measuring harmonics (NI
PXI-5671 and NI PXI-5660 operating in 300 KHz-2.7 GHz band and running
appropriate LabVIEW applications for RF CW generation and spectrum analysis).
Measurement Results
Figure 3 shows that the main resonances of probe (S22) are 880 MHz and
2640 MHz. Measured path loss S21 (-14 dB at 880 MHz, -32 dB at 1760 MHz,
and -30 dB at 2640 MHz) shows how the power received by the probe varies with
frequency (field strength inside cell remains constant during S21 measurement).
0
dB
-10
S21
880 MHz
-20
1760 MHz
2640 MHz
S22
-30
-40
0.5
1
1.5
2
Frequency (GHz)
2.5
3
Figure 3. Probe return loss (S22) and path loss (S21) of TEM cell.
12
AD-222
AD-223
AD-224
10
Pmin (dBm)
P3 (dBm) at 2640 MHz
Figure 4 shows that minimum power sensitivity thresholds in TEM cell at 880
MHz are 6 dbm, 1 dBm, and 3 dBm for AD-222, AD-223, and AD-224
accordingly. For all tags, 3rd harmonic power increases with input power.
8
6
4
2
0
Turn on points
-2
-60
AD-222
-65
AD-223
-70
AD-224
-75
-80
Noise level
-85
-90
860
870
880
890
Frequency (MHz)
900
-5
-2
1
4
7
Pin (dBm) at 880 MHz
10
Figure 4. Minimum power needed to activate tags in TEM cell (left) and the power of
third harmonic as a function of input power at 880 MHz (right).
Table 1 shows that harmonics are present even in the absence of tag (due
to transmitter spectrum). The 1st harmonic from the tag destructively interferes
with the cell field, lowering P1 by 2 dB. No significant 2nd harmonics due to tags
were observed. The 3rd harmonic distortion can be calculated as
HD3 = ( P3 / P1 ) ⋅ ( S 211 / S 213 ) and gives -41 dB for AD-222. 3rd harmonic field
(V/m) at probe location can also be easily found as E3 = (1 / h ) ⋅ ( P3 Z / S 213 ) .
Harmonic power
P1 (dBm)
Transmitter OFF
-91
No tag
-4
AD-222
-6
AD-223
-6
AD-224
-6
P2 (dBm)
P3 (dBm)
-90
-72
-72
-72
-72
-87
-86
-63
-64
-68
Table 1: Measured powers of harmonics for 10 dBm 880 MHz input signal.
Discussion and Conclusions
An RFID tag consists of an antenna and a chip and is usually designed to
operate in a specific frequency band where inductive impedance of the antenna is
close to complex conjugate of capacitive impedance of the chip. This causes the
tag to act as a resonance system with filtering properties. Most tags have dipolelike antennas which can resonate at odd harmonics and scatter those as well, while
scattering of even harmonics is suppressed, as we observed in measurements. All
tag backscatter emissions can potentially be mitigated by adding a dedicated
bandpass filter to the tag antenna.
We conclude that current passive UHF RFID tags can generate and scatter
harmonics and intermodulation products whose powers depends on specific tag
details (chip and antenna), as well as power and spectral content of interrogation
signal. Harmonic scattering from RFID tags can be characterized in controlled
EM environment such as TEM cell as shown in this paper.
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